Thomas was 15 at the time he was charged, which gave a judge the option to keep him in juvenile court or allow prosecutors to pursue the charges in adult court. A defendant has to be 16 years old to be automatically waived to adult court.

Court documents filed Wednesday allege that Thomas, who was previously identified in court documents as J.T. due to his age, was the shooter in the botched marijuana robbery.

"I just shot this guy," a witness heard Thomas say, according to court documents. "I have no remorse."

Indianapolis police were called to the 1900 block of South Union Street around 8 p.m. Feb. 2 after a resident said a man covered in blood knocked on the door. Officers followed a blood trail to a nearby home, where they found Arnett dead in a downstairs bathtub, shot in the face.

Thomas and another teenage defendant, Ahmond Hubbard, then 16, were arrested following an hours-long SWAT standoff in the early morning of Feb. 3. Hubbard was charged as an adult for his role in the shooting and faces one count each of murder and robbery resulting in serious bodily injury.

Hubbard had arranged to buy marijuana from Arnett via Instagram, according to court documents. Witnesses said Hubbard grabbed the drugs from Arnett and instructed the driver to leave the scene.